No abstract

In the United States over 1.7 million cases of traumatic brain injury are reported yearly, but predictive correlation of cellular injury to impact tissue strain is still lacking, particularly for neuronal injury resulting from compression. Given the prevalence of compressive deformations in most blunt head trauma, this information is critically important for the development of future mitigation and diagnosis strategies. Using a 3D in vitro neuronal compression model, we investigated the role of impact strain and strain rate on neuronal lifetime, viability, and pathomorphology. We find that strain magnitude and rate have profound, yet distinctively different effects on the injury pathology. While strain magnitude affects the time of neuronal death, strain rate influences the pathomorphology and extent of population injury. Cellular injury is not initiated through localized deformation of the cytoskeleton but rather driven by excess strain on the entire cell. Furthermore we find that, mechanoporation, one of the key pathological trigger mechanisms in stretch and shear neuronal injuries, was not observed under compression.

Mechanobiology relates cellular processes to mechanical signals, such as determining the effect of variations in matrix stiffness with cell tractions. Cell traction recorded via traction force microscopy (TFM) commonly takes place on materials such as polyacrylamideand polyethylene glycol-based gels. Such experiments remain limited in physiological relevance because cells natively migrate within complex tissue microenvironments that are spatially heterogeneous and hierarchical. Yet, TFM requires determination of the matrix constitutive law (stress-strain relationship), which is not always readily available. In addition, the currently achievable displacement resolution limits the accuracy of TFM for relatively small cells. To overcome these limitations, and increase the physiological relevance of in vitro experimental design, we present a new approach and a set of associated biomechanical signatures that are based purely on measurements of the matrix's displacements without requiring any knowledge of its constitutive laws. We show that our mean deformation metrics (MDM) approach can provide significant biophysical information without the need to explicitly determine cell tractions. In the process of demonstrating the use of our MDM approach, we succeeded in expanding the capability of our displacement measurement technique such that it can now measure the 3D deformations around relatively small cells (∼10 micrometers), such as neutrophils. Furthermore, we also report previously unseen deformation patterns generated by motile neutrophils in 3D collagen gels.traction force microscopy | confocal microscopy | large deformations | neutrophil M echanical cues within the cellular microenvironment regulate numerous fundamental functions including cell adhesion, deformation, and generation of traction (1-6). Analysis of cellular force generation, and its role in regulating homeostasis across a variety of cellular phenotypes and experimental platforms, has received much attention over the last three decades (7-13). Experimental quantification of cellular forces has produced several cell traction measurement techniques, ranging from surface wrinkle detection and flexure of micropillars to traction force microscopy (TFM) (12,(14)(15)(16)(17)(18)(19)(20). In TFM, measured cell-induced displacements are converted into tractions using various mathematical frameworks (14,15,17,18,21,22). Both twoand 3D TFM techniques have steadily increased in sophistication and now feature high-spatial displacement resolution and advanced computational formalisms to connect this displacement information to complex material constitutive laws (17,23,24).To successfully perform TFM, it is critical to know the stressstrain constitutive behavior of the matrix surrounding the cell. Although many TFM substrates feature relatively simple artificial gel constructs, such as polyacrylamide and polyethylene glycol, these constructs are impenetrable by cells and obviate measures obtained while cells are in a 3D setting (as would be the case within a bodily ...